The present invention relates to improvements in and relating to the construction of a metal-halogen (e.g. ZnBr.sub.2) electrolyte circulation-type cell stack secondary battery and more particularly to a cell stack secondary battery of the type in which the installation locations of a pair of micro-channels formed in the frame member of each framed electrode means or separating means for making uniform the flow of an electrolyte are selected on the right and left sides of the frame member thereby further stabilizing the flow of the electrolyte.
A typical example of the conventional metal-halogen electrolyte circulation-type cell stack secondary battery is disclosed in the specification of U.S. Pat. No. 4,461,817 assigned to the same assignee as the present application. The disclosed zinc-bromine secondary battery comprises, as a basic component unit of the secondary battery cell stack, a unit cell divided into a positive electrode chamber and a negative electrode chamber by an ion-permeable micro-porous membrane (separator), a positive electrode arranged in the positive electrode chamber and a negative electrode arranged in the negative electrode chamber. A positive electrolyte is circulated from a positive electrolyte storage tank through the positive electrode chamber by a positive pump and a negative electrolyte is circulated from a negative electrolyte storage tank through the negative elect chamber by a negative pump. With the two electrolytes being circulated in the described manner, during the charge the reactions of the following formulas take place:
At the negative electrode: Zn.sup.++ +2e.fwdarw.Zn PA0 At the positive electrode: 2Br.sup.- .fwdarw.Br.sub.2 +2e
In this case, bromine molecules (Br.sub.2) are produced at the positive electrode and mixed in the positive electrolyte. While a part of these molecules is dissolved in the positive electrolyte, the greater part is converted to a bromine complex compound by the bromine complexing agent in the positive electrolyte and the resulting complex compound is precipitated and stored in the positive electrolyte storage tank. During the discharge, with the electrolytes being circulated as mentioned previously, the reactions reverse to the previously mentioned formulas take place at the negative and positive electrodes and electric energy is discharged to an external load from the electrodes.
Where this zinc-bromine secondary battery is used for electric power storage purposes, for example, the electric power load leveling by storing with surplus electric power in nighttime and by discharging the stored electric power in daytime or as a power source for electric motor vehicles, generally an interruption period of at least more than eight hours is provided before starting the next discharge after the charge. During the interruption period, in order to prevent any self-discharge due to the diffusion of the dissolved Br.sub.2 in the positive electrolyte from the positive electrode chamber to the negative electrode chamber through the separator, it is necessary to flow out the positive and negative electrolytes from the respective battery cells (the positive and negative electrode chambers) to the respective electrolyte storage tanks and introduce a substituting air into the respective cells.
As a result, the substituted air in the cell must be exhausted before the discharge and thus it is necessary to utilize for example gravity so that in the battery service position the positive and negative electrolytes are gradually filled into the cell upward from the below and the light air is exhausted from the upper manifolds to the outside of the cell.
On the other hand, during the discharge Zn.sup.++ ions are produced on the negative electrode by a reaction of Zn Zn.sup.++ .fwdarw.+2e and these Zn.sup.++ .fwdarw.ions in the negative electrolyte are high in concentration thus tending to sediment in the negative electrode chamber. Therefore, the Zn.sup.++ ions must immediately be flowed out to the outside of the negative electrode chamber and for this purpose it is necessary to arrange so that in the battery service position the electrolyte is caused to flow upward from the below at an increased flow velocity or the electrolyte is caused to flow downward from the above. On the other hand, during the charge Br.sub.2 is produced on the positive electrode by a reaction of 2Br.sup.- .fwdarw.Br.sub.2 +2e and this Br.sub.2 in the positive electrolyte also tends to sediment in the positive electrode chamber. Thus, in order to flow out such Br.sub.2 to the outside of the positive electrode chamber or to supply such Br.sub.2 uniformly onto the lower surface of the positive electrode during the discharge, it is necessary to cause the electrolyte to flow downward from the above in the positive electrode chamber and thereby prevent the sedimentation of Br.sub.2.
As a result, when exhausting the substitute air in the cell by the positive and negative electrolytes prior to the discharge, the flow directions of the electrolytes must be reversed from those during the charge and discharge and for this purpose it is necessary to reverse the directions of rotation of the positive and negative pumps or arrange a four-way cock for reversing the flow directions of the electrolytes within the circulation loops.